1. Field of the Invention
The present invention relates generally to a mobile communication system employing an orthogonal frequency division multiplexing (OFDM) scheme, and in particular, to an apparatus and method for minimizing a peak-to-average power ratio (PAPR).
2. Description of the Related Art
Currently, mobile communication systems are rapidly progressing from a 3rd generation (3G) mobile communication system to a 4th generation (4G) mobile communication system. The 4G mobile communication system is being standardized to secure efficient interworking between a wired communication network and a wireless communication network and providing a wire/wireless integrated service rather than a simple wireless communication service provided in the earlier generation mobile communication systems. Therefore, it is necessary to develop advanced technology capable of transmitting mass data, whose quantity approaches the capacity of a wire communication network, even in a wireless communication network.
Therefore, in the 4G mobile communication system, research is being conducted on an orthogonal frequency division multiplexing (OFDM) scheme for high-speed data transmission in wire/wireless channels. The OFDM scheme, as a scheme transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) scheme that converts a serial input symbol stream into parallel symbols and then modulates the converted parallel symbols with a plurality of orthogonal subcarriers, i.e., a plurality of subcarrier channels, before transmission.
In a transmitter of an OFDM communication system, input data is modulated with subcarriers through a scrambler, an encoder, and an interleaver. The transmitter provides a variable data rate, and its coding rate, interleaving size, and modulation scheme are determined depending on the data rate. Commonly, the encoder uses a coding rate of ½ and ¾, and an interleaver's size for preventing a burst error is determined according to the number of coded bits per symbol (NCBPS). QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM are used for the modulation scheme. A predetermined number of pilot subcarriers are added to a signal modulated with a predetermined number of subcarriers by the above components, and then created into one OFDM symbol through an IFFT (Inverse Fast Fourier Transform) block. A guard interval for removing inter-symbol interference (ISI) in a multipath channel environment is inserted into the OFDM symbol, and then, finally input to an RF (Radio Frequency) processor through a symbol mapper. The RF processor RF-processes the input signal and transmits the RF signal in the air.
In a receiver of the OFDM communication system, a reverse process of the transmitter's process is performed, and a synchronization process is added thereto. First, a process of estimating a frequency offset and a symbol offset of a received OFDM symbol using a preset training symbol must be performed. Thereafter, a guard interval-removed data symbol is restored through an FFT (Fast Fourier Transform) block into a predetermined number of subcarriers to which a predetermined number of pilot subcarriers are added. In addition, in order to cope with a path delay phenomenon on an actual wireless channel, an equalizer estimates a channel condition for a received channel signal and removes signal distortion on the actual wireless channel from the received channel signal. The data channel-estimated through the equalizer is converted into a bit stream, deinterleaved by a deinterleaver, and output as final data through a decoder for error correction and a descrambler.
The above-described OFDM scheme is characterized by maintaining orthogonality between subcarriers during transmission, thereby obtaining optimal transmission efficiency during high-speed data transmission. Moreover, since overlapped frequency spectrums are used, the OFDM scheme has high frequency efficiency, and is robust against frequency selective fading and multipath fading. Further, the OFDM scheme can reduce an influence of inter-symbol interference by using a guard interval, and makes it possible to design an equalizer using simple hardware. Further, the OFDM scheme is robust against impulse noises. Currently, the OFDM scheme having such advantages is actively applied to high-speed, high-capacity data communication systems, such as an IEEE (Institute of Electrical and Electronics Engineers) 802.16a communication system and an IEEE 802.16e communication system.
The OFDMA (Orthogonal Frequency Division Multiple Access) communication system must use a signal having a low peak-to-average power ratio (PAPR) in order to guarantee its normal system performance. The reason for using a signal with a low PAPR will be described below.
The OFDMA communication system, which is a multicarrier communication system, uses a plurality of carriers, i.e., a plurality of subcarriers, so orthogonality of each of the subcarriers is considered important. Therefore, a phase is set such that orthogonality should be kept between the subcarriers, and in a process of transmitting/receiving signals through the subcarriers, when the phase is changed, the subcarrier signals may overlap. In this case, a level of the signals overlapped due to the phase change deviates from a linear region of an amplifier included in the OFDMA communication system, making normal signal transmission/reception impossible. Therefore, the OFDMA communication system uses a signal having a minimum PAPR.
As stated above, PAPR minimization in the OFDM communication system is an important factor in improving system performance, so a large amount of research is being carried out on a scheme for minimizing the PAPR. The scheme for reducing the PAPR includes a clipping scheme, a block coding scheme, and a phase adjustment scheme. A description will be made herein below of the schemes for reducing the PAPR.
1) Clipping Scheme
In the clipping scheme, when a level of a signal exceeds a preset level, a signal component exceeding the preset level is clipped away to thereby reduce PAPR. Because the clipping scheme simply clips away the signal component exceeding the preset level, its implementation is very simple. However, the clipping scheme is disadvantageous in that in-band distortion occurs due to non-linear operation, increasing a bit error rate (BER), and inter-neighbor channel interference occurs due to out-band clipping noises.
2) Block Coding Scheme
In the block coding scheme, a coding scheme is applied to a redundancy subcarrier in order to reduce PAPR of all subcarrier signals. The block coding scheme, as it applies the coding scheme, has an error correction capability and further, can reduce PAPR without signal distortion. However, when the total number of subcarriers is large, its spectrum effect is very poor and a look-up table or a generation matrix increases in size, disadvantageously causing an increase in complexity.
3) Phase Adjustment Scheme
The phase adjustment scheme is classified into two schemes: a selective mapping (SLM) scheme and a partial transmit sequence (PTS) scheme. The SLM scheme multiplies the same length-N data by M statistically-independent length-N sequences, selects a sequence having a minimum PAPR among the M resultant sequences, and transmits the selected sequence. The PTS scheme divides a length-N data block into M subblocks, performs an (L+P)-point IFFT operation on each of the M subblocks, multiplies each of the M (L+P)-point IFFT-transformed subblocks by a phase parameter for minimizing PAPR, and then, sums up the multiplied results before transmission. Advantageously, the SLM scheme and the PTS scheme can efficiently reduce PAPR. However, an IFFT operation process for each of the M subblocks is added, undesirably causing an increase in complexity.
In the block coding scheme for reducing PAPR, a code rate is defined as
  R  =            k      N        =                  k                  2                      k            -            1                              .      Here, R represents a code rate, N represents the total number of subcarriers for the OFDM communication system, and k represents the number of input information data bits. When the total number N of subcarriers for the OFDM communication system is 256 (N=256), a code rate in the block coding scheme has a very low value of 9/256=0.035. Accordingly, there is a demand for a scheme for reducing PAPR while resolving the low code rate and low bandwidth efficiency problems, which are major disadvantageous of the block coding in the OFDM communication system.